The Zong Massacre occurred on Nov. 29, 1781. The Zong, a slave ship, was headed towards Jamaica when the captain gave the order to throw 54 enslaved Africans overboard.
Here is a description from PBS Africans in the Americas.
Another 78 were drowned over the next two days. By the time the ship had reached the Caribbean, 132 persons had been murdered. When the ship returned to England, the owners wished to be compensated the full value for each enslaved African lost.
The claim might have been honored if it had not been Olaudah Equiano (also known as Gustavus Vassa), who had once been enslaved. While living in England, he learned of the tragedy and alerted an abolitionist friend. The case went to court.
At first the jury ruled in favor of the ship’s owners. Since it was permissible to kill animals for the safety of the ship, they decided, it was permissible to kill enslaved people for the same reason. The insurance company appealed, and the case was retried.
This time the court decided that the Africans on board the ship were people.
This is the case featured in the film Belle. Historian James Walvin wrote a detailed account in The Zong: A Massacre, the Law and the End of Slavery.